Freeciv

Freeciv is a free turn-based multiplayer strategy game, in which each player becomes the leader of a civilization, fighting to obtain the ultimate goal:
To become the greatest civilization.

Players of Civilization II® by Microprose® should feel at home, since one aim of Freeciv is to have compatible rules.
Freeciv is maintained by an international team of coders and enthusiasts, and is easily one of the most fun and addictive network games out there!

The goal of Freeciv is to build cities, which in turn can build armies to attack the other players. You get 2 settlers and 1 explorer. Using the numeric keypad, move the settlers to a good location and build a city by pressing b. You can move diagonally, by the way. Freeciv is turn based; every unit can move once per turn. The turn ends when everyone clicks "Turn done" or when a timeout is over.